After years of debate, state transportation officials Thursday announced a final plan to build the biggest highway project in Boston in a generation: putting the Massachusetts Turnpike at ground level in Allston and elevating a section of Soldiers Field Road along the Charles River onto a new viaduct above the highway.

The $1.1 billion project promises to radically reshape the western face of the city and disrupt travel for years on a stretch of highway that carries about 150,000 cars a day. The main objectives are to replace the aging viaduct and straighten the turnpike where it takes a sweeping curve through now-abandoned rail yards. That in turn would free up land for a new train station and dozens of acres of new development by Harvard University, as well as creating a wider band of green space along the river.

The start of construction is at least several years off — the state still does not yet know how it will even pay for the job, and intends to set a funding strategy after the final design and price tag are set.

While the planning for the turnpike rebuild has long been in the works, state officials spent months mulling options for the narrow section between the Boston University Bridge and Cambridge Street colloquially called “the throat.”

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Currently, the eight-lane highway towers over the Charles on a viaduct above commuter rail tracks. Four-lane Soldiers Field Road, which serves as a continuation of Storrow Drive, runs next to the highway at grade level, essentially reducing the adjacent path for cyclists and walkers to a thin strip along the shoreline.

On Thursday, Transportation Secretary Stephanie Pollack announced the current alignment will essentially flip, with the turnpike at grade level next to the railroad tracks, and Soldiers Field Road elevated over either the eastbound or westbound highway lanes.

“It strikes the best balance between all the different transportation objectives,” Pollack said. “Moving the state highway down to grade, that was very important to a lot of stakeholders. And while there is still a viaduct to carry a portion of Soldiers Field Road, it will be a less intrusive viaduct.”

DAVID L. RYAN/GLOBE STAFF/FILE 2017

Originally, the state planned to keep the highway on a viaduct but agreed to consider other options at the insistence of neighbors and advocates who preferred it be rebuilt at ground level.

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Another option involved having both roadways at ground level, but state officials feared that approach would run afoul of environmental permitting related to building along the water. They also considered putting the railroad on a viaduct, even elevating the bike path.

The option to elevate Soldiers Field Road only emerged in the last few months, from a team of consultants hired by the state. Pollack said it “can be permitted expeditiously, best balances the present-day and future mobility needs of all users, and protects and enhances as best as possible the natural and historic resources and public realm of the Charles River Basin and neighboring Allston community.”

The new viaduct would add about 20 feet to the strip of parkland along the river, creating a larger path, and allow for more pedestrian bridges between Commonwealth Avenue and the Boston University campus and the Charles.

Rick Dimino, president of A Better City, a nonprofit that has pushed for the at-grade highway, called the decision “a collective victory for all of us.” He hopes the state will place the viaduct as far back from the shoreline as possible, above the eastbound side of the turnpike.

The administration of Mayor Martin J. Walsh also cheered the decision, saying in a statement it will lead to more open space and “a better experience for everyone who uses this essential corridor.”

Activists were also pleased, but said the next steps will be equally important.

“This design allows footbridge connections from the BU campus and Comm. Ave. to the Charles. Are those going to be built, or not?” said Harry Mattison, an Allston resident who has served on a working group convened by the state. “You have lots and lots of questions like that that we look forward to working with MassDOT to figure out.”

Amy Laura Cahn, an attorney with the Conservation Law Foundation, which also supported raising Soldiers Field Road, said the state must now protect the Charles during construction.

“Even in picking the best possible option, we still need to make sure the plan for that option addresses potential environmental and climate harms,” she said.

The project could take eight years to build, Pollack said, but the daily commute of tens of thousands of drivers probably won’t be affected until 2021 at the earliest, after the state completes environmental planning and hires a contractor.

Much is not known at this point, including which sections of the highway will be closed, for how long, and how wide the disruptions will extend to surrounding roads and commuter rail trains. Pollack did say that this version, with the Soldiers Field Road viaduct, is more complicated because it requires relocating utilities and other infrastructure.

Some effects are already known: the bike path will likely close for some periods, as will the railroad bridge over the Charles the MBTA uses to relay commuter rail cars on the south side of the city to a big repair facility on the north. Facing a possible four-year shutdown, the MBTA will need to find a new way to service those south side trains, as the only alternative path to the repair facility is a circuitous 120-mile trip west past Worcester.

Still, in outlining her decision, Pollack said the benefits of the new construction will outweigh all the disruptions.

Officials will spend the next year determining the exact location of the new viaduct and construction methods to minimize disruptions.

And in another major development, Pollack also directed officials to study how quickly a new commuter rail stop on the Harvard-owned land, called West Station, can be built. The state’s original forecast for a 2040 opening for West Station was lambasted by neighborhood activists, Harvard, BU, and others, who said that new development on the site will require better transit options. Harvard has offered $58 million to help build the station, including $8 million for an early, no-frills version.

Ari Ofsevit, of the advocacy group Transit Matters, said activists must now focus on getting West Station built quickly.

“West Station, from the advocates’ point of view, is the next big fight and the next big step,” Ofsevit said. “The turnpike’s full. We can’t add more cars to it.”